- Volume 22, Issue 1, 1986

A family of small plasmids encoding resistance to nucleic acid-binding (NAB) compounds has recently been identified in strains of Staphylococcus aureus isolated in Italy, Texas and Western Australia. The mol. wts of the NAB-resistance plasmids are in the range (1.5–1.9) × 106 and all but one encode resistance to acridine yellow, ethidium bromide and quaternary ammonium compounds. The largest of the plasmids, pWG1773, differed in that it did not confer resistance to ethidium bromide. Restriction enzyme analysis of these plasmids revealed four distinct patterns corresponding to plasmids of four different mol. wts and physical maps were constructed based on the restriction patterns. Two plasmid types of molecular sizes approximately 2440 and 2240 base pairs had a 610-base pair region in common. Physical maps of the other two plasmid types were not related. The presence of a family of small NAB-resistance plasmids which carry no other known phenotypic markers provides further evidence for the strong selective advantage associated with maintenance of this determinant in clinical isolates of S. aureus.

Interactions between trophozoites of Entamoeba histolytica and peritoneal exudate macrophages from unsensitised and antigen-sensitised animals were studied in vitro. Normal macrophages killed trophozoites to some extent. This killing capacity was enhanced by prior sensitisation of the animals with specific antigen. Incorporation of anti-amoebic antiserum in the amoeba-macrophage mixture greatly enhanced the killing capacity of macrophages. Fraction one (F-I) of a crude amoebic extract was most effective in enhancing the cytotoxicity of macrophages by prior sensitisation and anti-F-I serum was the most effective antiserum. The cytotoxicity-inducing capacity of the immune serum resided in the IgG but not in the IgM fraction.

Dot hybridisation with 32P- and biotin-labelled probes prepared from leptospiral DNA was performed to develop a sensitive and specific diagnostic method for early infection with leptospires. The smallest amounts of leptospiral DNA that could be detected with 32P- and biotin-labelled probes was 1.5 pg and 5 pg, respectively, corresponding to about 750 and 2500 leptospires. Dot hybridisation with a 32P-labelled probe detected leptospiral DNA in sera from all of 14 experimentally infected golden hamsters. The smallest amount of leptospiral DNA detected in these experiments corresponded to about 2500 leptospires. In the test conditions described in this study, the sensitivity of dot hybridisation with a biotin-labelled probe was lower. Little cross-hybridisation was observed with unrelated DNAs which indicates that dot hybridisation could be a useful diagnostic method.

A cell-culture assay was used to detect toxins directly in stools from sporadic cases of infantile diarrhoea. Cytotoxins were revealed in 11 out of 58 samples from children with diarrhoea, nine of whom had no common enteric pathogens in their stools. A preliminary characterisation of the cytotoxins was obtained by neutralisation tests with clostridial antitoxins.

The kinetics of spore production by Clostridium difficile were not paralleled by release of C. difficile toxin A in vitro. Toxin A was not found to be associated with either purified whole spores or spore coats. Residual traces of toxin A detected in spore contents were almost certainly derived from contaminating vegetative cell debris. Thus, toxin A is unlikely to be a spore constituent or associated with sporogenesis.

The time course and nature of mucosal damage induced in rabbit ileal loops by two strains of Salmonella typhimurium (TML and W118) isolated from human infections was assessed by immunofluorescence microscopy and by scanning and transmission electronmicroscopy. Salmonella-induced fluid secretion occurred in the presence or absence of gross mucosal architectural damage. Neither strain caused mucosal ulceration. When damage did occur, the villi were shortened by loss of their tip regions with concomitant reforming of an intact mucosal surface. Immediately preceding the onset of fluid secretion, marked infiltration of the mucosa with polymorphonuclear leukocytes and occasional macrophages was seen. This revives an earlier suggestion that interaction between invading salmonellae and acute inflammatory cells may be an important factor in initiation of fluid secretion. Brush-border invasion by salmonellae cannot per se be the immediate cause of fluid secretion, because the latter occurred several hours after initial invasion.

The biochemical characteristics, enterotoxigenicity and susceptibility to antibiotics are reported for 22 strains of Aeromonas species isolated from clinical specimens in the Western Region of Saudi Arabia. Aeromonas caviae was the species most frequently observed; a high proportion of these strains fermented lactose, whereas lactose fermentation was not observed in strains of A. hydrophila and A. sobria. Enterotoxigenicity, as judged by cytotoxicity in tissue culture was observed in three of four A. hydrophila strains and six of seven A. sobria strains, but in only one of 11 A. caviae strains. Two schemes for the biochemical assessment of enterotoxigenicity were found to be in 91% and 86% agreement respectively with cytotoxicity studies and in 95% agreement with each other. No single biochemical test correlated fully with enterotoxigenicity, but 86% of strains that oxidased gluconate produced a cytotoxin. Most strains were inhibited by concentrations of gentamicin, amikacin, chloramphenicol and tetracycline achievable in plasma. Most strains were resistant to broadspectrum penicillins and many were also resistant to cefuroxime and cefoxitin.

Different cellular proteins of the gastric campylobacter-like organism (GCLO) were shown to be immunogenic for man. Antibodies to GCLO were detected in sera by both complement fixation and enzyme-linked immunoabsorbent assay. Antibody was found in 133 (52%) of 254 patients attending for gastroscopy. There was a high correlation between presence of antibody and a positive GCLO culture from the gastric mucus. Patients with normal endoscopic appearances, duodenal ulcer, duodenitis and oesophagitis had similar prevalences (c. 50%) of antibody. Only patients with endoscopically visible gastritis or gastric ulcer had a higher frequency (c. 80%) of antibody. In a normal population, antibody was uncommon in individuals <20 years old, but the prevalence of antibody increased (to c. 50%) with age. There was little evidence to support an important pathological role for GCLO in disorders of the upper gastrointestinal tract, although the possibility that it may be a co-factor in the pathogenesis of gastric ulcer cannot be excluded.

The pathogenicity of Neisseria gonorrhoeae is a subject of considerable interest. It is believed that N. gonorrhoeae of colonial type 1 are pathogenic while those of type 4 are not. This is based on experimentation in human volunteers. The object of this study was to determine the reasons for the differences of susceptibility of chicken embryos to N. gonorrhoeae strains of colonial types 1, 4, 5 and 1R (a type-1 revertant from a non-pathogenic type 4 strain originally tested in human volunteers). Colonial types 1, 5, 1R and 4 caused mortality rates of 80, 70, 85 and 20% respectively. This variation in lethality appeared to depend upon the availability of free extra-cellular endotoxin and this was confirmed by chicken-embryo inoculation results and electronmicroscopy of normal and heated colonial types 1, 4 and 5. Similar results were obtained by inoculating purified endotoxins from these types into chicken embryos. The results of this study suggest that endotoxins play a major role in the pathogenicity of N. gonorrhoeae and that the variations in virulence of the colonial types depends on the stability of their cell walls.

A cluster of infections caused by gentamicin-resistant gram-negative bacilli in a District General Hospital was investigated. The plasmids coding for gentamicin resistance in the infecting organisms and in isolates obtained from the ward environment and from the faeces of patients and staff were characterised. Six plasmids encoding gentamicin resistance were recognised amongst the organisms causing infection. Two of the plasmids were found in different serotypes of the same species and one plasmid was found in different genera. Three of the plasmids present in the organisms causing the infections were also present in the inanimate environment or in the bowel flora of patients and these also were found in different serotypes of the same species and in different genera.

Chloramphenicol resistance (Cmr) plasmids pSK2 and pSK5 from Staphylococcus aureus and pSK 102 and pSK 103 from S. epidermidis have been characterised and detailed restriction endonuclease cleavage maps constructed. TaqI digestion profiles illustrated the identity of pSK 5 and pSK 102 and also revealed a high degree of similarity between these four Cmr plasmids from Australian staphylococci and three Cmr plasmids from S. aureus strains of geographically unrelated origin. DNA-DNA hybridisation indicated that the chloramphenicol acetyltransferase determinant carried by pSK 5/pSK 102 could be found on other structurally-distinct Cmr plasmids. The role of S. epidermidis as a reservoir for Cmr plasmids found in S. aureus is discussed.

Repeated subcultures were performed on 572 blood cultures that had yielded single significant organisms and 55 from which contaminants had grown. This yielded 83 more isolates from 71 patients. Only one of the contaminated blood cultures yielded a further significant isolate. Additional antibiotic therapy was indicated in only six patients. Although repeated subculture increased the detection of additional isolates, it did not help in the management of most patients.